This is the answer I posted on Hugh Barnes' question, slightly revised to fit the context here...
I am a strong believer in the value of the "explain your changes" field. Yes, it's possible to click on each of the edits in a change history to figure out when a particular addition or deletion was made, and by whom, but a good summary of the edit makes it far easier to understand a profile's history. I try to explain all of my changes, but I confess that I sometimes fail to do so -- particularly if I'm making a inconsequential edit, such as fixing a typo in an edit I made 30 seconds earlier.
I'd like to offer a compromise approach that would greatly increase the utilization of the "explain your changes" field. Wikipedia has a feature (it's an opt-in feature for registered users, so you won't encounter it if you make anonymous edits there) that prompts people to add an edit summary before saving a change. If I try to save an edit there without explaining my change, I see a message box that says
Reminder: You have not provided an edit summary. Edit summaries help other users understand the intention of your edits. Please enter one before you click Save page again, or your edit will be saved without one.
In my experience at Wikipedia, that's been very effective in ensuring that I explain my changes. I believe that a similar reminder here -- not a mandatory requirement to add an edit summary, but just a nag -- would cause many more contributors to get into the habit of explaining (or at least describing) their changes.
Note: At Wikipedia, users who don't use the edit summary are viewed unfavorably when they seek extra editing privileges or positions of responsibility in the community.